Monolithic integrated micro-supercapacitors with ultra-high systemic volumetric performance and areal output voltage

ABSTRACT Monolithic integrated micro-supercapacitors (MIMSCs) with high systemic performance and cell-number density are important for miniaturized electronics to empower the Internet of Things. However, fabrication of customizable MIMSCs in an extremely small space remains a huge challenge considering key factors such as materials selection, electrolyte confinement, microfabrication and device-performance uniformity. Here, we develop a universal and large-throughput microfabrication strategy to address all these issues by combining multistep lithographic patterning, spray printing of MXene microelectrodes and controllable 3D printing of gel electrolytes. We achieve the monolithic integration of electrochemically isolated micro-supercapacitors in close proximity by leveraging high-resolution micropatterning techniques for microelectrode deposition and 3D printing for precise electrolyte deposition. Notably, the MIMSCs obtained demonstrate a high areal-number density of 28 cells cm−2 (340 cells on 3.5 × 3.5 cm2), a record areal output voltage of 75.6 V cm−2, an acceptable systemic volumetric energy density of 9.8 mWh cm−3 and an unprecedentedly high capacitance retention of 92% after 4000 cycles at an extremely high output voltage of 162 V. This work paves the way for monolithic integrated and microscopic energy-storage assemblies for powering future microelectronics.


INTRODUCTION
For truly realizing the Internet of Things consisting of miniaturized electronics, microscale electrochemical energy-storage systems with high systemic performance, superior cell-number density, tunable capacitance and output voltage are required [1][2][3][4]. In this regard, on-chip interdigitated microsupercapacitors (MSCs) free of separators and external metal connection wires, simultaneously with reliable electrochemical performance and tunable connection, will greatly boost the cell-number density and systemic performance for monolithic integrated MSCs (MIMSCs) with desirable customizability in a limited space [5,6].
So far, enormous progress has been achieved for MSCs, including electrode materials engineering, high-precision microfabrication techniques, functional properties and their integration with microelectronic systems [7][8][9]. However, scalable production of fully-functioning compact MIMSCs with high systemic performance, superior cell-number density and tunable performance is still challenging, due to the difficulty in the precise deposition of electrolytes on densely-packed MSCs for electrochemical isolation, sacrifice in electrochemical performance during complex microfabrication procedures and limited performance uniformity among numerous individual cells in large-scale arrays. In particular, the footprint of a single cell and interspace between adjacent cells of most reported MIMSCs are too large, resulting in low systemic performance and cell-number density [10][11][12]. For instance, screen-printed MXene MIMSCs exhibited a low areal cell-number density of 1.7 cells cm −2 [13] and inkjet-printed MXene/PH1000 MIMSCs presented an areal cell-number density of 9 cells cm −2 [14]. Although some dense microelectrodes arrays have been constructed previously by lithography [15], focused ion beam patterning [16], laser scribing [17] and inkjet printing techniques [18,19], the functioning MIMSCs have not been achieved owing to the lack of an accurate electrolyte-deposition technology. Second, considerable attention should be paid to improving the electrochemical performance, which is usually sacrificed due to poor compatibility between electrode materials with multistep complex microfabrication procedures [20,21]. For example, MIMSCs fabricated via electrohydrodynamic jet printing demonstrated a high arealnumber density of 54.9 cells cm −2 . However, this delicate fabrication technique has severely limited the selection of materials, resulting in low areal capacitance of a single cell (127.5 μF cm −2 ) [22]. Third, the performance uniformity among numerous individual cells, including microelectrodes and electrolytes, determined by each step in the microfabrication processes, crucially affects the output voltage and cyclability of the resultant MIM-SCs [23,24]. Therefore, innovation in the whole microfabrication technique to develop MIMSCs that simultaneously achieve superior cell-number density and stable systemic performance is highly necessary.
Herein, we develop a universal and reliable strategy to achieve mass production of microelectrodes with high cell-number density and precise deposition of gel electrolytes. This is achieved by combining multistep lithography for defining the micropatterns, spray printing of MXene microelectrodes and 3D printing of quasi-solid-state gel electrolytes. Each individual MXene-based MSC (M-MSC) exhibited an extremely small footprint of 0.018 cm 2 , high areal capacitance of 4.1 mF cm −2 , volumetric capacitance of 457 F cm −3 and stable performance at ultra-high scan rate of ≤500 V s −1 . A novel 3D printing process was developed to deposit the designed gel electrolyte ink, resulting in the electrochemical isolation of adjacent M-MSCs in the monolithic array of M-MSCs (denoted as M-MIMSCs) and demonstration of outstanding performance uniformity. The resultant M-MIMSCs offered an exceptional areal cell-number density of 28 cells cm −2 (340 cells on 3.5 × 3.5 cm 2 ), systemic volumetric energy density of 9.8 mWh cm −3 , ultra-high output voltage of 200 V and the highest areal output voltage of 75.6 V cm −2 so far. This work provides an avenue for achieving super-dense arrays of MSCs for powering diverse microelectronic applications.

Fabrication of M-MIMSCs
The on-chip microfabrication of M-MIMSCs is illustrated in Fig. 1 and Supplementary Fig. S1. First, a thin photoresist (AZ4620) layer with predesigned micropatterns on a target substrate (e.g. Si, glass or flexible polyethylene terephthalate, Supplementary Fig. S2) was obtained by using the lithographic process. Next, Au/Ti (250 nm) was sputtered over the substrate followed by lift-off in acetone, forming current collectors for microelectrodes and electrical connections between adjacent microcells (Supplementary Fig. S3). Then, another photoresist layer with microelectrode patterns was obtained by the same lithographic process on Au/Ti current collectors ( Fig. 1b and Supplementary Fig. S4). In terms of the requirement for high-precision microelectrodes, an aqueous dispersion of small-size MXene (1-nm-thick nanosheet) with a lateral dimension of 100-600 nm ( Fig. 2a and b, and Supplementary  Fig. S5) was prepared by etching Ti 3 AlC 2 with LiF and HCl [13,25], followed by sonication. This was chosen as the active material owing to its high capacitance, metallic conductivity and solution-processing ability [15]. Subsequently, a MXene film (Supplementary Fig. S6) was printed on the entire surface by spray printing. Afterward, the MXene film deposited on the unexposed photoresist regions was removed through another lift-off process in acetone, creating high-resolution interdigital microelectrodes with a fine feature size of 100 μm and an extremely small footprint of 0.018 cm 2 ( Fig. 2ce). Satisfactorily, no residual active material was observed from the precise profile of the interdigital microelectrodes even after bath sonication used for the lift-off process (Fig. 2e), implying the strong adhesion between the metal surface and the MXene nanosheets. Further, 340 integrated M-MSCs with an adjacent interval of 600 μm were fabricated on a 3.5 × 3.5 cm 2 rigid substrate, achieving an unprecedented areal-number density of 28 cells cm −2 that far exceeds values from previous reports [11,14,23,24], exhibiting application potential in real situations requiring compact integration energy supply ( Fig. 1c and e, and Supplementary Fig. S7). Remarkably, M-MIMSCs with 340 cells fabricated on a flexible polyethylene terephthalate substrate weighed only 78 mg, which is critical for miniaturized robots unable to carry heavy power-supply units [26]. They also exhibited robust flexibility and stretchability ( Fig. 1f and g). Micrometer-sized electrodes with various customizable shapes could be readily constructed using predesigned masks, demonstrating the aesthetic diversity of this strategy (Fig. 2h). Finally, to guarantee the electrochemical isolation of adjacent microcells in close proximity, printable quasi-solid-state gel electrolyte inks were prepared ( Fig. 2g and h, and Supplementary Fig. S8) and then precisely deposited on microcells by using a reliable and controllable 3D printing technique (Figs 1d and 2j, Supplementary Fig. S9a and Supplementary Video S1). It is worth noting that the appropriate 3D printing mode is critical for the fabrication of high areal-cell-number-density and stable MIMSCs. As shown in Supplementary Fig. S10, under inappropriate printing modes, such as dot printing and line-segment printing, no matter how the printing parameters are adjusted, the gel electrolyte could not achieve complete coverage of the  and Supplementary Fig. S9c). Incomplete coverage of electrolyte on microcells resulted in an incomplete utilization of the microelectrodes and inferior systemic performance. Uneven coverage resulted in poor performance consistency of each microcell in the MIMSCs and hence poor cycle stability.

Superior performance of an individual M-MSC
To demonstrate their superior performance, we evaluated an individual M-MSC with a footprint of 0.018 cm 2 (microelectrode thickness of 90 nm) using a PVA/H 2 SO 4 polymer gel electrolyte. As expected, approximately rectangular cyclic voltammetry (CV) curves measured at different scan rates (Fig. 3a) and nearly linear symmetric triangular-shaped galvanostatic charge discharge (GCD, Supplementary Fig. S11) profiles were obtained, indicating typical capacitive behavior of MXene nanosheets. Remarkably, the linear dependence of the current response on the scan rate was ≤500 V s −1 (Fig. 3b), demonstrative of superior electronic and ionic conductivity of MXene microelectrodes, which was further confirmed by a low equivalent series resistance of 0.3 cm 2 (inset of Fig. 3c). Our M-MSC exhibited a low phase angle of −77.8 • at 120 Hz and the cross-over frequency at which the phase angle reached −45 • was 2153 Hz (Fig. 3c), close to commercial Al electrolytic capacitors [15]. It is strongly suggestive of the potential of M-MSCs for alternating current-line filtering. Furthermore, our M-MSC delivered satisfactory areal capacitance of 4.1 mF cm −2 and competitive volumetric capacitance of 457 F cm −3 at 10 mV s −1 (Fig. 3d), benefitting from abundant edge planes and superior electrochemical activity of small-sized MXene nanosheets. The areal capacitance could  Supplementary Fig. S14). It is worth noting that the maximal scan rates of our M-MSCs in both aqueous and ionic liquid electrolytes are superior to most reported MXene-based MSCs so far (Fig. 3g) [15,[27][28][29]. This prominent feature of our M-MSCs is attributed to the strong interfacial interaction of small-sized MXene nanosheets and highly conductive current collectors to remarkably boost fast charge transfer and rapid ion transport of the whole device.

Integration for ultra-high output voltage
To demonstrate performance uniformity and integration, M-MIMSCs comprising 340 cells with tailored integrated connections were fabricated on a small area (3.5 × 3.5 cm 2 , Fig. 4a). 3D thickness mapping (Fig. 4b) and high-magnification scanning electron microscopy (SEM) imaging (Supplementary Fig. S15) of a selected cell in Fig. 4a presented a flat and continuous surface texture, testifying to the uniformity and nondestructive nature of the spray-printing and lift-off process. Additionally, the thickness profiles of nine independent M-MSCs at different regions in the M-MIMSCs are shown in Fig. 4c, further indicating exceptional uniformity on a larger scale. All the CV curves of our M-MIMSCs  [24], inkjet-printed graphene MSC [10], inkjet-printed MXene/PH1000 MSC [14], electrohydrodynamic jet-printing nanoactivated carbon MSC [22], screen-printed MXene MSC [13], lithographically CNT MSC [34], screen-printed graphene [11] and laser-induced graphene MSC [23]). connected in series from 20 to 334 cells, obtained at 4.8 V s −1 in PVA/H 2 SO 4 electrolyte, displayed nearly rectangular shapes with typical capacitive behavior and accordingly a step-wise linear increase in the output voltage from 12 to 200 V (Fig. 4d). Such extraordinary tandem capacitive behavior was also demonstrated by GCD profiles with symmetrical triangular shapes, showing linearly increasing output voltage and invariable charge/discharge time ( Supplementary Fig. S16). Superior modularization and performance uniformity across each microcell were evidenced by a plot of the capacitance and voltage versus the cell number, which delivered a linear increase in the output voltage and a non-linear decrease in the capacitance with a correlation coefficient close to 1 (Fig. 4e). It is noted that 334 cells and 200 V are the highest integrated cell number and output voltage reported for serially integrated MSCs to date (Fig. 4f) [10,13,14,[22][23][24]34]. Meanwhile, the overall capacitance can be readily enhanced by customizable parallel connections of M-MIMSCs. As shown in Supplementary Fig. S17, 20 M-MSCs connected in series as a cell pack and multiple packs connected in parallel exhibited a simultaneous increase in output current, voltage and capacitance. Therefore, our strategy is highly customizable for producing monolithic integrated microscale energy storage with high output voltage and tailored capacitance to meet the varying customizable requirements in actual scenarios. Additionally, the stretchable M-MIMSCs were fabricated by introducing a honeycomb structure between the microcells on a flexible polyethylene terephthalate substrate (Fig. 1g, Supplementary Fig. S18 and Supplementary Video S2). Notably, the resultant M-MIMSCs could be stably charged and discharged under dynamic stretching to 225% without electrochemical performance degradation, exhibiting excellent mechanical stretchability.

CONCLUSION
In summary, we have developed a reliable and universal microfabrication strategy combining high-resolution lithographic patterning and sprayprinting techniques with elaborate control of 3D printing to achieve on-chip M-MIMSCs with a high areal-number density (340 cells on 3.5 × 3.5 cm 2 , 28 cells cm −2 ), output voltage of ≤200 V, areal output voltage of 75.6 V cm −2 , systemic volumetric energy density of 9.8 mWh cm −3 and long-term stability. Our strategy is expected to be applicable to other MIMSCs and integrated micro-batteries, which could achieve high modular output capacities. Further architectural design (such as microelectrode configuration) is also possible to improve the space utilization of monolithic micropower sources for compact integration and high-systemic-performance-requiring applications.

METHODS
The detailed preparation and characteristic methods of materials are available in the online Supplementary file.